Many steam stimulation, steam drive and gravity drainage schemes (and combinations thereof) have been suggested to produce in-situ bitumen. Designers of these schemes have typically sought to promote the mass transfer of the steam into the areas of the reservoir in which bitumen is thought to be present. Steam stimulation typically involves drilling wells into an underground bitumen deposit and using the wells sequentially for steam injection and then for production, which is called a steam soak cycle. Mass transfer of steam into the deposit may be promoted by fracturing the reservoir. Steam drive and gravity drainage schemes make use of steam injection wells and production wells simultaneously. Mass transfer of the steam can take the form of a communication path or steam breakthrough from the injection well(s) to the production well(s). The flow of steam warms the adjacent bitumen and encourages it to flow toward the production well(s). Alternatively the steam is encouraged into the deposit to create a steam chamber that grows from the injection well(s) to the production well(s). In any event, leakage of steam to a basal water transition zone or elsewhere in the reservoir where the bitumen saturation level is low (hereinafter called "thief zones") has been avoided as being a waste of heat energy and as being unproductive.
Canadian patent specification 2,015,459 discloses a process for confining steam injected into a heavy oil reservoir having a thief zone, wherein a pressurized non-condensable gas is injected into the thief zone to inhibit the escape of injected steam through that zone.
U.S. Pat. No. 4,344,485 and Canadian patent specification 1,304,287 disclose steam assisted gravity drainage processes wherein steam is injected via an upper horizontal well section to transfer heat to the normally immobile heavy oil so that it will melt and will drain by gravity to a lower horizontal well section where the oil is recovered.
U.S. Pat. No. 4,390,067 discloses the use of a rectangular grid of horizontal steam injection wells to create heated corridors in a viscous oil or bitumen bearing formation from which viscous oil or bitumen is then produced via vertical production wells.
U.S. Pat. No. 4,702,314 discloses an oil production system comprising a rectangular four spot production well pattern and a vertical steam injection well at the centre of the pattern, wherein the production wells comprise horizontal inflow sections that point towards the steam injection well.
U.S. Pat. No. 4,283,088 discloses a thermal oil mining method wherein a series of steam injection and oil production wells is drilled in an upward direction and in a star-shaped configuration into the oil bearing formation from a ring-shaped working tunnel which is located near the bottom of said formation.
It is an object of the present invention to provide a method and system for in-situ production of bitumen which promotes the mobility of bitumen in larger volumes of the bitumen bearing formation than has been previously possible.